660 Crankcase Volume

[NPKenny]

I have noticed a bit of talk recently on how the crankcase volume contributes to the saws overall performance. Specifically in the Reverse Engineering of 7900 (which looks to have disappeared) thread. Upon personal investigation Dolmar looks to have a significantly larger crankcase volume.

I have to pull apart the 660 crankcase to replace the crank bearings and was wondering if I should do anything such as polish or grind any areas to maximize the crankcase efficiency; mixing, flow, etc. After cleaning, is there value in fresh paint in the crankcase or is no paint better?

Additionally, I am contemplating a 660BB kit for the top since the bad crankshaft bearings trashed the intake side of the piston and nicely scored the jug. Will the extra volume of the BB kit be affected in any way by the crankcase volume and is there anyway to maximize this set-up.

I've done a good bit of search on crankcase volume and the BB set-up, but not nearly as much info as I hoped for for the crankcase.

Thanks to all.

 

[blsnelling]

Do not paint the inside of the case. It will dissolve in the gasoline. Remove anything's that loose and leave it like that.

Many have said that the BB is no better than the OEM. I'd prefer the quality of OEM myself.

 

[kevlar]

No! Don't polish the cases! been there done that, does not work! Now waiting for a good used case to come my way.Same saw 066.

 

[brncreeper]

With regard to crank case volume attributing to performance there may be more to it than just that. At “the other site” it has been known for some time now that a ported 7900 and a ported 372 (75cc) produce similar (almost equal) performance. So wouldn’t the 372 have a smaller crank case volume than the 7900?

 

[Urbicide]

Timberwolf would be a good source of knowledge here......opcorn:

 

[kevlar]

actually my thread is still up in the hot saw forum under crank case post and polish-wish I would have posted before I did the work!

 

[NPKenny]

No! Don't polish the cases! been there done that, does not work! Now waiting for a good used case to come my way.Same saw 066.

Good to know! I am glad I asked ahead.

 

[djohn]

Reduced crankcase volume creates higher crankcase pressure which should cause the crankcase mixture to rush through the transfer ports faster getting more mixture in the combustion chamber. It works on bikes with expansion chamber pipes, not so sure it would be effective on a saw without much of an expansion chamber. Also too much reduction in volume is bad as well so you shouldn't just fill'er up.

Thoughts?